Preheat your oven to 375°F (190°C) and line a baking sheet with parchment paper.
Brush the cut sides of the acorn squash with olive oil and season with salt and pepper.
Place the squash cut-side down on the baking sheet and roast for about 25 minutes until tender.
While the squash is roasting, heat olive oil in a pan over medium heat. Sauté the onion and garlic until translucent.
Add the mushrooms and cook until they release their moisture and start to brown.
Stir in the spinach until wilted, then remove from heat and mix in the cooked quinoa, thyme, and sage.
Once the squashes are roasted, fill them with the mushroom and spinach mixture.
Return to the oven and bake for another 15 minutes.
Serve warm as a satisfying vegan main course.